The invention concerns a magnet configuration with a superconducting magnet coil system and a current path which is at least partially superconducting and comprises at least parts of the magnet coil system, wherein at least two electric connecting points are provided on the current path, which define at least one section within the current path, wherein the section does not comprise all parts of the magnet coil system contained in the current path, and at least one resistive element with a resistance of ≧0 Ohm, and at least two electric contacts, wherein the electric contacts are disposed between the resistive element and one of the at least two electric connecting points.
A magnet configuration of this type comprising a magnet coil system that is divided into several sections is disclosed e.g. in U.S. Pat. Nos. 6,307,370, 6,369,464 and 6,777,938.
Superconducting magnet coil systems are usually configured from several partial-coils. Partial coils, to carry the same electric current during operation, are typically connected in series and form one single superconducting current path. This is advantageous in that the partial coils can be charged and discharged simultaneously using one single current source, with no undesired current differences developing between the partial coils during charging and discharging of the magnet coil system, such that the currents need not be matched during operation.
It is sometimes however, advantageous to eliminate the series connection of the partial coils by providing branchings at certain points in the current path using an electric switching element, which is connected in parallel. The configuration of U.S. Pat. No. 6,307,370 represents a first example of a magnet coil system with branchings in the superconducting current path. The superconducting current path of this configuration is divided into sections, which are bridged via protective resistors. If the superconductivity in a partial coil suddenly fails (quench), the operating current can be discharged from this section via the protective resistor. This prevents the coil from being damaged by high electric voltages or excessive heating.
A change in the surrounding magnetic field, in which the magnet configuration is embedded, is another situation in which it is advantageous for the current path of the magnet coil system to have branchings. Due to the principle of magnetic flux conservation, a current is thereby induced in the superconducting coils of the magnet configuration, which generates a counter field for changes in the surrounding field. When all partial coils are combined into one single current path, individual partial coils may impede each other during compensation of the field disturbance or the disturbance may be overcompensated. Compensation of a disturbance in the surrounding field is achieved when the individual partial coils of the magnet configuration can react to the disturbance with an individually varying current change. This is realized e.g. in a configuration according to U.S. Pat. No. 6,369,464, wherein a section of the superconducting current path of the magnet coil system is bridged and short-circuited by a superconducting switch.
The subject matter of U.S. Pat. No. 6,307,370 is also based on the improvement of the capability of the magnet coil configuration to compensate for field disturbances. Apart from its primary function as quench protection elements, the protective resistors are thereby disposed and designed in such a manner that they allow induction of different compensation currents in different partial coils when the surrounding field changes. This prevents mutual impairment of individual partial coils during compensation of the field disturbance as well as overcompensation of the disturbance. In contrast to U.S. Pat. No. 6,369,464 the differences in the compensation currents, which flow through the protective resistors, decay with time.
In addition to quench protection and disturbance compensation, there is another situation in which branchings in the superconducting current path of the magnet coil system are advantageous. This case is described in U.S. Pat. No. 6,777,938. The superconducting current path thereby comprises a resistive faulty location which results in a magnetic field drift if the operating current is not supported by a current source. This problem is solved in that a non-resistive section of the current path is superconductingly short-circuited and has an inductive coupling with the resistive section. While the slightly resistive section is slowly discharged, the superconductingly short-circuited section is continuously inductively charged. This can maintain the overall field of the configuration at a constant value (drift compensation).
One problem with using branchings in the superconducting current path of a magnet coil system is that, during charging and discharging of the magnet coil system, currents flow through the electric components in contact with the branchings. When the resistance values of these elements are small, it may take a very long time until the currents in the coil sections are balanced at the end of the charging process. Moreover, the components that are connected in parallel at the coil sections are heated. This is critical when the components are located in the cooling container of the magnet system. Since superconducting magnet coil systems must be cooled down to very low temperatures of typically a few Kelvin, discharging of additional heat is very expensive.
For this reason, the resistance values of protective resistors, which are permanently connected to the superconducting current path, must be sufficiently high. If the resistance values are too high, the protective resistors can no longer provide ideal protection in case of a quench. Resistance values of the order of magnitude of one Ohm are usually a good compromise for dimensioning protective resistors.
When a partial section of the superconducting current path is to be short-circuited for compensating a magnetic drift or magnetic disturbances, resistance values on the order of magnitude of one ohm are too large. For such applications, the short-circuit is ideally in the extremely low-ohmic or superconducting range. These short-circuits are usually designed as a superconducting switch instead of a low-ohmic resistive element or a superconducting connection. By heating the superconducting wire of this switch is brought into a resistive state during charging and discharging of the magnet configuration, such that a high-ohmic connection is produced between the connecting points of the superconducting switch instead of a superconducting short-circuit. This partially prevents branching of current from the superconducting current path of the magnet coil system during charging and discharging. However, heating of the superconducting switch introduces heat into the cooling container of the magnet configuration that causes undesired loss of coolant.
In contrast thereto, it is the underlying purpose of the present invention to provide branchings for the current in the superconducting current path in the superconducting operating state of a magnet configuration with a superconducting magnet coil system and a superconducting current path, which comprises at least parts of the superconducting magnet coil system, at the same time preventing branching of the currents from the superconducting current path during charging and discharging of the magnet configuration.